Review: Chemical aspects of the use of gold clusters in structural biology

The gold clusters used as labels in structural biology are organometallic compounds formed by a tightly packed core of gold atoms surrounded by a shell of phosphine ligands. Certain features distinguish them from conventional organic compounds. The main characteristic of significance for their use in biology is the fluxionality of the ligands. Both intra- and intermolecular exchange of ligands can occur; and in addition, the phosphine ligands may exchange with mercapto-compounds, leading to a sensitivity particularly of large gold clusters to SH compounds. The rate of ligand exchange in undecagold clusters is in the range of days at room temperature and therefore of little significance for their use as labels. In contrast, large (Au55) gold clusters display a very fast ligand exchange, making it impossible to prepare pure cluster species with just one reactive group. These clusters can nevertheless be used for covalent attachment to biological substances as well, as demonstrated by the many successful experiments published. The possibility of crosslinking or of spontaneous interaction with SH groups should, however, not be ignored in the interpretation of experimental results.